Search

There is increasing concern over the impact of atmospheric nitrogen (N) deposition on forest ecosystems in the tropical and subtropical areas. In this study, we quantified atmospheric N deposition and revealed current plant and soil N status in 14 forests along a 150 km urban to rural transect in southern China, with an emphasis on examining whether foliar δN can be used as an indicator of N saturation. Bulk deposition ranged from 16.2 to 38.2 kg N ha yr, while the throughfall covered a larger range of 11.7-65.1 kg N ha yr. Foliar N concentration, NO leaching to stream, and soil NO concentration were low and NO production was negligible in some rural forests, indicating that primary production in these forests may be limited by N supply. But all these N variables were enhanced in suburban and urban forests. Across the study transect, throughfall N input was correlated positively with soil nitrification and NO leaching to stream, and negatively with pH values in soil and stream water. Foliar δN was between −6.6‰ and 0.7‰, and was negatively correlated with soil NO concentration and NO leaching to stream across the entire transect, demonstrating that an increased N supply does not necessarily increase forest δN values. We proposed several potential mechanism that could contribute to the δN pattern, including (1) increased plant uptake of N-depleted soil NO, (2) foliage uptake of N-depleted NH, (3) increased utilization of soil inorganic N relative to dissolved organic N, and (4) increased fractionation during plant N uptake under higher soil N availability. [ABSTRACT FROM AUTHOR]